1. Field of the Invention
This invention relates to apparatus and methods for transferring developer materials from a source to multiple areas of a latent image containing surface. More particularly, the present invention is concerned with magnetic brush roll apparatus and methods for transferring magnetically attractable toner carrying materials from a source into contact with multiple areas of a latent image carrying surface. The invention is especially useful in conjunction with plain paper copying machines requiring development of latent electrostatic images by use of magnetizable carriers and triboelectrically compatable toner powders or the like.
2. Description of the Prior Art
Conventional electrophotography copiers typically employ a sequence of steps including the charging of a photoconductor surface such as a plate, drum, or belt, the exposure of this surface to light reflected from a document to be copied, the application of developer materials to the photoconductor surface, the transfer of these developer materials to the paper and subsequent fusing of the developer materials along with other housekeeping steps associated with the photoconductor. Some prior art copier devices introduce the developer materials to the photoconductor surface by pouring or similar steps. Other prior art copiers use toner carrying mixes comprised of magnetizable or magnetically attractable carrier elements such as plastic encased steel beads and triboelectrically compatable toner powder carried by these beads. This type of toner carrying mix has made it possible to apply the toner to the photoconductor in a brush like manner by means of magnetic brush rollers. Typically, these rollers include a magnet configuration for adhering the toner carrying mix to the surface of a rotating roller cylinder and with the magnetic field being coextensive through a zone between the source of the toner material and the area in proximity to the photoconductor surface wherein developer application occurs.
Increased process speeds for faster operating copiers has necessitated higher mix flow rates and wider area coverage of exposed photoconductor surface to supply sufficient toner particularly required to develop gray and half-tone images with the copy quality available through lower process speeds. Unfortunately, the efficiency of single roll magnetic brush developers is relatively low, such as on the order of one percent in terms of the amount of toner used to the amount delivered in the mix. Accordingly, various prior art devices have been developed for the purposes of applying developer containing materials to several areas or a larger area of the photoconductor surface so as to increase the efficiency of toner transfer and allow increasing speeds of copier operation. Generally, such multiple magnetic brush configurations employ magnetic field generating magnets within the rollers which effect direct transfer of unused toner containing materials from one roller onto the next roller. In some instances, this transfer has been augmented by use of scraper plates and the like. In many such multiple magnetic brush systems, either the brush rollers which apply the toner carrying material to the latent image drum surface are arranged so as to directly engage a sump or other source of the materials or else the rollers are arranged in a series with alternate such rollers being positioned to act merely as conveyors rather than to provide any useful function in applying toner carrying material to the image drum surface. The alternate conveyor brush roller devices suffer the disadvantage of requiring greater physical space and thus are primarily limited in utility to photoconductor strips as contrasted to photoconductor drums.
Although some prior art multiple brush roller apparatus has been developed wherein all rollers apply toner carrying materials to the photoconductor surface, such prior art apparatus requires special interconnecting magnet field configurations between the rollers such that the toner exiting from the initial roller is completely attracted by the subsequent roller. Thus, there has been no ability to control the amount of toner transferred from the initial brush roller to subsequent rollers.